What is an indicator of low hydraulic fluid in a truck's system?

A spongy or unresponsive brake pedal is indeed an indicator of low hydraulic fluid in a truck's braking system. In hydraulic brake systems, fluid is essential for transferring force from the brake pedal to the braking mechanism at the wheels. When hydraulic fluid levels are low, there can be insufficient pressure to properly engage the brakes, leading to a spongy feel or unresponsiveness when the pedal is pressed.

This sensation occurs because the brake system relies on a sealed, pressurized line of hydraulic fluid to operate effectively. Low fluid can result in air entering the system or a failure to transmit force efficiently, severely compromising braking performance and safety. Monitoring hydraulic fluid levels is crucial for maintaining optimal brake function and ensuring vehicle safety.